Many kinds of electronic flash apparatus for photographic purposes are known in the art. The present invention relates to the type of flash apparatus which is often referred to as a computing or computer flash, that is, a unit having means responsive to light reflected from the object being photographed, to terminate the flash when sufficient light for a good exposure has been produced, thus avoiding the over-exposure that would result if the flash were allowed to continue for its maximum duration, using up all the energy stored in the storage capacitor. The invention further relates to such flash units having (as above said) an exposure metering and flash limiting device, and further having a discharge circuit comprising a storage capacitor and a flash tube and an electronic switch contained in the discharge circuit, the flash tube and switch becoming conductive with time delay, and the switch element which becomes conductive later being arranged behind the switch element which becomes conductive earlier, with reference to the direction of flow of the discharge current.
In such flash units, the exposure metering and flash limiting device is customarily provided with a feed voltage only during the phase of the illuminating or flashing of the flash tube. The purpose of this is to prevent the exposure metering and flash limiting mechanism from responding to strong light radiations from other sources of light or from external flashes and thus prematurely interrupting the flash discharge of the electronic flash device, leading to underexposure of the photographic picture.
In one known electronic flash device of this type, the supplying of the exposure metering and flash limiting device with current takes place by means of a capacitor which is charged in the "ready" condition of the electronic flash device, and is shunted in series with a resistor over the series circuit of the flash tube and electronic switch. At the moment of the firing of the flash tube and of the electronic switch (conveniently developed as a thyristor) the above mentioned capacitor discharges over the flash tube, the thyristor, and the resistor, and the drop in voltage at the resistor forms the supply voltage for the exposure metering and flash limiting mechanism. In order to obtain a constant feed of voltage for the exposure metering and flash limiting device, a Zener diode is furthermore connected in parallel with the resistor. A resistor is connected in the discharge circuit of the capacitor, in order to limit the current.
An object of the present invention is to provide an electronic flash apparatus having an exposure metering and flash limiting device or mechanism, which is proof against external flash within the meaning explained above, and which is characterized by an extremely simple construction, using a minimum number of switch elements.